Dimeric cyclic esters such as lactide are polymerizable to high molecular weight poly(hydroxycarboxylic acids) which are of great interest for their hydrolytic and biodegradable properties. For example, they have long been of interest for such biomedical uses as sutures and staples. More recently, they have become of interest for the manufacture of articles of commerce for non-biomedical uses that would be degradable in the environment, in particular hydrolytically, to environmentally acceptable products. For most, if not all such uses, it is preferred the degradable polymer be made from dimeric cyclic ester. However, dimeric cyclic esters made by existing technology are too costly for such non-medical uses because of low yields, by-product formation and cumbersome processing.
The preparation of the dimeric cyclic esters of alpha-hydroxycarboxylic acids is an old and much studied process. Heretofore, the preparation has been conducted in two generally distinct batch steps involving first preparing an oligomer of the hydroxy carboxylic acid, i.e., a relatively short-chain condensation polymer thereof, then heating the polymer under reduced pressure to generate the desired cyclic ester. Methods for producing a cyclic ester are discussed in the following references: Gruter et al., U.S. Pat. No. 1,095,205 (1914); Lowe, U.S. Pat. No. 2,668,162 (1954); Bellis, U.S. Pat. No. 4,727,163 (1988); Bhatia, U.S. Pat. No. 4,835,293; Muller, Ger. patent application Publication Nos. 36 32 103 and 37 08 915 (1988). Such processes spanning over 70 years of technology suffer in that they require hours of reaction time at high temperatures for the conversion of the polymeric intermediate to the cyclic ester. Further, the rather long residence times at the high temperatures employed often result in side reactions, leading, for example, to unwanted isomers, charring of the polymer and consequently difficult to handle reactor heels.
It will be noted the Muller German patent application Publications, mentioned above, disclose continuous as well as batch operations for the depolymerization of lactic acid oligomers to lactide where the conversion takes several hours to complete. Muller states his process can be run batchwise, continuously or semi-continuously. These processes, however, depend strongly on greatly reduced pressures for the removal of the desired lactide from the reaction zone. Maintaining such low pressures is not only expensive on a commercial scale but does not solve the problems associated with long residence times. Also, product recovery under vacuum makes downstream processing cumbersome as many of these esters are solid at room temperature. The product obtained has large amounts of impurities, requiring solvent washing and recrystallization; and activated carbon treatment to remove impurities.
My previously issued patent (U.S. Pat. No. 4,835,293) mentioned above discloses a gas-assisted depolymerization process which improves upon the herein above-listed processes of the art.
It is an object of this invention to provide a further improved gas-assisted process at low residence times and at high conversion rates for converting polymers (oligomers) of alpha-hydroxy carboxylic acids or their esters or nitrogen base salts to cyclic esters at high production rates. It is a particular object to provide such a process for producing lactide from the appropriate oligomer of lactic acid or ester of lactic acid or salt of lactic acid.